The present invention relates to the field of wireless communications systems, and in particular, to a method and system for selecting between transmission modes of a communication device to improve performance.
One advance in increasing the capacity of communication systems has been in the area of resource sharing or multiple access. Examples of multiple access techniques include code division multiple access (CDMA), frequency division multiple access (FDMA), and time division multiple access (TDMA). For example, in a TDMA system, each remote user terminals communicates with a hub communication device (e.g., a base station) in a frequency channel shared with other remote user terminals, but in its own (i.e., non-overlapping) time slot. As such, in a TDMA system, multiple remote user terminals may communicate with the hub communication device within the same frequency channel, but within non-overlapping time slots. (The term xe2x80x9cchannelxe2x80x9d as used herein refers to any one or a combination of conventional communication channels, such as frequency, time, code channels).
Although antennas have sometimes been one of the neglected components of wireless systems, relatively recent development in the field of (adaptive) antenna arrays and xe2x80x9csmart antennasxe2x80x9d have not only realized significant improvements in geographic capacity, but also in signal-to-noise ratio (SNR), interference reduction, directionality, spatial diversity, power efficiency, and security. Accordingly, employment of antenna arrays has been proposed in a number of different wireless applications, including, but not limited to, radio communication systems, cellular systems, television broadcasting, paging systems, medical applications, etc.
Antenna arrays typically include a number of antennas that are spatially separated and coupled to one or more processors. Adaptive antenna arrays, or simply, adaptive arrays, periodically analyze the signals received from each of the antennas in an array to distinguish between one or more desired signals (e.g., from a desired source, such as cellular telephone or other remote user terminal) and one or more undesired signals (e.g., interference from remote user terminals sharing the same or adjacent frequency, interference from other radio frequency (RF) emissions overlapping in channel, Johnson noise, multipath, other interference sources, etc). In doing so, adaptive array systems generally compute uplink and downlink xe2x80x9cweights,xe2x80x9d which include information about how to transmit (in the case of a downlink weight) and how to receive (in the case of an uplink weight) to diminish gain in the direction of one or more interfering sources, while enhancing gain in the direction of a desired source. Thus, the weight values describe the uplink and/or downlink beamforming strategy for adaptive array systems. It should be appreciated herein that adaptive array systems may be switched beam or adaptive, wherein the latter case has a virtually infinite number of beamforming patterns that can be varied adaptively based on changing signal environment.
Because adaptive array systems may sometimes be able to distinguish between spatially distinct sources (e.g., two cellular user units at different locations), such systems are sometimes referred to as xe2x80x9cspatial processingxe2x80x9d or xe2x80x9cspatial division multiple access (SDMA)xe2x80x9d systems. In general, adaptive array systems provide relatively significant improvement in performance relative to single antenna element systems.
FIG. 1A is a diagram depicting a simplified radiation pattern of an antenna array system, according to the prior art. In the system shown in FIG. 1A, an antenna array 10 transmits (downlink) signals to and/or receives (uplink) signals from a desired source 12, such as a mobile or stationary remote user terminal (e.g., a cellular voice and/or data communication device, a PDA having wireless capability, etc.). As shown, a beamforming pattern 8, which represents the transmission and/or reception directional gain pattern (depicted for only two dimensions) for the antenna array 10, includes a region of enhanced gain 6, as well as a region of relatively minimized gain or xe2x80x9cnullxe2x80x9d region 2 and another region of relatively minimized gain or null 4.
The null regions 2 and 4 represent one of the advantages of adaptive arrays and xe2x80x9csmart antennaxe2x80x9d processing. In particular, each of the nulls 2 and 4 represent a represent a region or direction of relatively minimized gain with respect to the beamforming pattern of the antenna array 10. As such, the antenna array 10 typically directs a null in the direction of an interfering source. To this end, the null 2 is directed toward an interfering source 14, while the null 4 is directed to the interfering source 16. The interfering sources 14 and 16 each may represent a moving car, another mobile or stationary remote user terminal in communication with the antenna array 10 or another communication device (e.g., a base station that may or may not include an antenna array), etc., which, may cause interference. As such, null generation may be viewed as interference mitigation, and each xe2x80x9cnull regionxe2x80x9d may be referred to as an interference mitigated region.
By enhancing the gain in the direction of desired source, while diminishing (and ideally reducing to zero) the gain in the direction of one or more interfering sources, the antenna array 10 may xe2x80x9cdirectionallyxe2x80x9d receive and transmit signals, and as such, increase system capacity, decrease interference to the desired source(s), etc.
FIG. 1B is a graphical representation of a beamforming pattern for the antenna array 10 shown in FIG. 1A, according to the prior art. In FIG. 1B, the level of the transmission (downlink) and reception (uplink) gain of the antenna array 10 is depicted on the vertical axis and (spatial) direction is shown on the horizontal axis. As shown, there is relatively greater gain in the direction of the desired source 12, which corresponds to the enhanced gain region 6, than there is toward the interfering source 14, which corresponds to the null region 2, or the interfering source 16, which corresponds to the null region 4.
It should be appreciated that the term xe2x80x9cnullxe2x80x9d as used in the context of adaptive array systems does not necessarily mean, and often does not mean, a region of zero electromagnetic energy, since nulls may often include some level of gain, though typically less than the enhanced region 6. Ideally, a communication device employing an adaptive array would direct a null having zero gain toward an interfering source. Furthermore, the closer the gain value of a null is to zero, the more intense or xe2x80x9cdeepxe2x80x9d the null is. Thus, the xe2x80x9camount of nullingxe2x80x9d that an antenna array or adaptive array system may generate may be defined one or both the number of nulls and the intensity of such nulls, such that the greater the amount of nulling, the greater the number of nulls generated and/or the more intense/deep one or more nulls are. As shown in FIG. 1B, for example, the null 2 is less intense than the null 4, since the latter represents less gain (and greater nulling intensity).
Unfortunately, performing interference mitigation (or xe2x80x9cnullingxe2x80x9d) generally diminishes the overall transmit and/or receive power of an antenna array system. In general, the number of nulls an adaptive array can generate is limited. For example, if a base station employing an antenna array directs relatively maximum power to a desired remote user terminal, such that a few, none, or relatively less intense nulls are generated, other remote user terminals, either in communication with the base station or another base station, may experience interference because of the relatively high transmission power used to communicate with the desired source. The interference may be especially problematic in systems where one or more base stations support spatial channelsxe2x80x94i.e., when two or more remote user terminals may simultaneously share the same carrier frequency and/or time slot for communication with the base station. On the other hand, if the base station uses a downlink beamforming strategy that directs relatively less gain toward the desired remote user terminal, for example, due performing a relatively greater amount of nulling, then the desired remote user terminal may suffer from fading or other types of performance degradation due to insufficient transmit power by the base station, even though the effect interference to other sources may be diminished.
Thus, what is desired is method and system that overcomes the above-mentioned or other effects associated with performing interference mitigation in adaptive array systems. THIS PAGE INTENTIONALLY LEFT BLANK
The present invention relates to a method and apparatus for providing selective interference mitigation (or nulling) in an adaptive array system. In accordance with one embodiment of the invention, a communication device, typically employing an adaptive array, determines an indication of signal reception quality for a first remote user terminal in communication with the communication device. The communication device provides a first mode of operation wherein a first amount of nulling with respect to at least one interfering source is generated by the communication device. In addition, the communication device provides a second mode of operation characterized by generating a second amount of nulling with respect to the at least one interfering source, wherein the second amount of nulling is relatively greater than the first amount of nulling. Based on the relative signal reception quality indication, the communication device selects one of the first and second modes of operation.